Method for evaluating a crystalline semiconductor substrate

ABSTRACT

In a method for evaluating a semiconductor crystal substrate which includes a collector layer, a base layer, and an emitter layer and is used for a heterojunction bipolar transistor, a semiconductor crystal substrate to be evaluated which includes a crystal layer whose composition is the same as that of the base layer is produced. Excitation light is irradiated to the semiconductor crystal substrate to be evaluated and a change with time in an intensity of photoluminescence from the crystal layer is measured before the intensity becomes saturated. A change with time in a current gain of the heterojunction bipolar transistor produced using the semiconductor crystal substrate is measured based on the change with time in the intensity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for evaluating asemiconductor crystal substrate, and more particularly to a method forevaluating a semiconductor crystal substrate which includes a collectorlayer, a base layer, and an emitter layer and is used for heterojunctionbipolar transistors.

[0003] 2. Background Art

[0004] Heterojunction bipolar transistors (hereinafter referred to asHBTs) are widely used for power amplifiers for portable telephones, etc.since they provide good high-frequency characteristics and high currentdensity. As its emitter-base junction, the HBT employs a heterojunctionin which the emitter layer has a band gap larger than that of the baselayer to enhance the emitter injection efficiency of the bipolartransistor. A semiconductor device made up of HBTs employs asemiconductor crystal substrate having a multilayer structure.

[0005] With reference to FIG. 9, a description will be made of a generalcross-sectional structure of the semiconductor crystal substrate forHBTs, using an AlGaAs HBT as an example. As shown in the figure, theAlGaAs HBT includes a semiconductive GaAs substrate 32, and an n-GaAssubcollector layer 33, an n-GaAs collector layer 34, a p-GaAs base layer35, an n-AlGaAs emitter layer 36, and an n-GaAs contact layer 37, whichare all formed on the semiconductive GaAs substrate 32 in that order.These layers are formed by epitaxially growing each layer by use of, forexample, the metalorganic chemical vapor deposition method (hereinafterreferred to as the MOCVD method). Further in FIG. 9, reference numeral38 denotes collector electrodes; 39 denotes base electrodes; and 40denotes an emitter electrode. The collector electrodes 38 have alaminated structure made up of, for example, AuGe/Ni/Au. The baseelectrodes 39, on the other hand, have a laminated structure made up of,for example, Pt/Ti/Au. Furthermore, the emitter electrode 40 is made of,for example, WSiN.

[0006] To enhance the high-frequency characteristics of an HBTconfigured as described above so that its characteristics are sufficientfor a microwave device, the base resistance must be reduced by reducingthe thickness of the p-type compound semiconductor crystal layerconstituting the base layer and increasing the impurity concentration.For example, a known method for reducing the base resistance is to add,as an impurity, carbon to the p-GaAs layer, which is a p-type compoundsemiconductor crystal layer used as the base layer, in order to increasethe carrier concentration of the base layer. In this method, however,hydrogen is undesirably taken into the base layer from ambientatmosphere in the base layer growth process. If hydrogen is includedinto the base layer, an initial change in the electricalcharacteristics, especially in the current gain is observed, which isdisadvantageous to the quality control. This phenomenon is explainedbelow using a specific example.

[0007]FIG. 10 shows the change in the current gain (β) of an HBT withchanging base current (Ib). The HBT indicated by the figure has a baselayer whose carrier concentration and hydrogen concentration areapproximately 4×10¹⁹ cm⁻³ and 2×10¹⁹ cm⁻³, respectively. The thicknessof the base layer of the HBT is approximately 1,000 Å, and its emittersize is 4×20 μm. The change in the current gain (β) was measured fivetimes on the same conditions. In the figure, the label “Firstmeasurement” indicates the characteristic curve measured for the firsttime immediately after the device was produced, while the label “Fifthmeasurement” indicates the characteristic curve measured for the fifthtime.

[0008] As shown in FIG. 10, the current gain (β) changes with changingbase current (Ib). Specifically, when the base current (Ib) isincreased, the current gain (β) increases to a certain value and thendecreases. The shapes of the curves of the current gains (β) measuredimmediately after energization for the first time and the fifth time aregreatly different from each other when the current gains (β) increase.Specifically, the current gain (β) increases more rapidly as the numberof times the device is energized increases. However, the maximum valueof the current gain (β) measured for the first time is not largelydifferent from that measured for the fifth time. Furthermore, the shapesof the curves obtained when the current gains (β) decrease aresubstantially the same.

[0009] The occurrence of the phenomenon shown in FIG. 10 that thecurrent gain increases more rapidly with increasing number ofenergization operations is conceivably attributed to hydrogen includedin the base layer of the HBT. That is, inclusion of hydrogen into thebase layer of the HBT makes the electrical characteristics of the deviceextremely unstable, which is disadvantageous to the quality control ofthe semiconductor device. On the other hand, the change in the currentgain with increasing number of energization operations becomes small forthe fifth and later measurements, making the characteristics stabilized.However, inspecting the product after its characteristics have becomestable takes considerable time, which is not preferable in terms ofproductivity.

[0010] Furthermore, conventionally, it is difficult to measure aninitial change in the current gain at the time point when the crystalhas been grown. That is, it is not possible to determine the initialchange in the current gain until an HBT device is actually manufactured(from the grown crystal) and its electrical characteristics areevaluated. Such characteristics (as the current gain change) of asemiconductor crystal substrate cannot be determined without actuallymanufacturing an HBT device from it, raising the problem that it is notpossible to perform the quality control at stages before the HBT ismanufactured from the semiconductor crystal substrate.

SUMMARY OF THE INVENTION

[0011] The present invention has been devised in view of the aboveproblems. It is, therefore, an object of the present invention toprovide a method for evaluating a semiconductor crystal substrate insuch a way that it is possible to estimate the initial change in thecurrent gain of the semiconductor crystal substrate.

[0012] Another object of the present invention is to provide a methodfor evaluating a semiconductor crystal substrate in such a way that itis possible to perform quality control of an HBT device.

[0013] Other objects and advantages of the present invention will becomeapparent from the following description.

[0014] According to one aspect of the present invention, in a method forevaluating a semiconductor crystal substrate which includes a collectorlayer, a base layer, and an emitter layer and is used for aheterojunction bipolar transistor, a semiconductor crystal substrate tobe evaluated which includes a crystal layer whose composition is thesame as that of the base layer is produced. Excitation light isirradiated to the semiconductor crystal substrate to be evaluated and achange with time in an intensity of photoluminescence from the crystallayer is measured before the intensity becomes saturated. A change withtime in a current gain of the heterojunction bipolar transistor producedusing the semiconductor crystal substrate is measured based on thechange with time in the intensity.

[0015] Other and further objects, features and advantages of theinvention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1A is a change in a base current with time depending on aconcentration of hydrogen.

[0017]FIG. 1B is a change in a base current with time depending on aconcentration of hydrogen.

[0018]FIG. 1C is a change in a base current with time depending on aconcentration of hydrogen.

[0019]FIG. 2A shows across section of a semiconductor crystal substrateused for PL evaluation.

[0020]FIG. 2B shows a cross section of a semiconductor crystal substrateused for PL evaluation.

[0021]FIG. 3A is a change in a PL intensity with time depending on aconcentration of hydrogen.

[0022]FIG. 3B is a change in a PL intensity with time depending on aconcentration of hydrogen.

[0023]FIG. 4A shows the relationship between a base current and a PLintensity.

[0024]FIG. 4B shows the relationship between a base current and a PLintensity.

[0025]FIG. 4C shows the relationship between a base current and a PLintensity.

[0026]FIG. 5 shows a cross section of a semiconductor crystal substrateaccording to the first embodiment.

[0027]FIG. 6 shows across section of a semiconductor crystal substrateaccording to the second embodiment.

[0028]FIG. 7 shows across section of a semiconductor crystal substrateaccording to the third embodiment.

[0029]FIG. 8 shows across section of a semiconductor crystal substrateaccording to the fourth embodiment.

[0030]FIG. 9 shows a cross section of a HBT device.

[0031]FIG. 10 shows the relationship between a base current and acurrent gain.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Preferred embodiments of the present invention will be describedbelow with reference to the accompanying drawings.

[0033] The change in the current gain of an HBT shown in FIG. 10 isrelated to the hydrogen concentration in the base layer. Thisrelationship will be described in detail with reference to FIG. 1A˜FIG.1C. FIGS. 1A, 1B, and 1C each shows the change in the base current withtime. Specifically, FIG. 1A shows the change in the base current withthe concentration of the hydrogen contained in the base layer set to1×10¹⁹ cm⁻³; FIG. 1B shows the change with the concentration set to 4×10¹⁸ cm⁻³; and FIG. 1C shows the change with the concentration set to1×10¹⁸ cm⁻³. All of the curves shown in FIG. 1A˜FIG. 1C were obtainedwith the collector-emitter voltage and the base-emitter voltage fixed to2.5V and 1.3V, respectively. As shown in the figures, the higher thehydrogen concentration, the larger the change in the base current withtime. The current gain (i), the base current (Ib), and the collectorcurrent (Ic) are related by the following formula,

β=ΔIc/ΔIb.

[0034] It should be noted that a similar phenomenon is observed inphotoluminescence (hereinafter abbreviated as PL) evaluation, which willbe described below in detail.

[0035] PL is a light-emitting phenomenon which occurs when minoritycarries (electrons in the case of a p-type semiconductor) within asemiconductor recombine with holes (or electrons) and thereby formelectron-hole pairs after the minority carriers are excited byirradiating to the semiconductor a light having a wavelength with energylarger than the forbidden energy band gap.

[0036]FIGS. 2A and 2B each shows a semiconductor crystal substrate usedfor PL evaluation. The semiconductor crystal substrate shown in FIG. 2Acomprises a GaAs substrate 1, and an i-GaAs layer 2, ani-In_(0.5)Ga_(0.5)P layer 3 (200 Å thick), a C-doped p-GaAs layer 4(carrier concentration: 4×10¹⁹ cm⁻³, thickness: 1,000 Å), ann-In_(0.5)Ga_(0.5)P layer 5 (carrier concentration: 3×10¹⁷ cm⁻³,thickness: 200 Å), and an i-GaAs layer 6 (200 Å thick), which are allformed on the GaAs substrate 1 in that order. The semiconductor crystalsubstrate shown in FIG. 2B, on the other hand, comprises a GaAssubstrate 7, and an i-GaAs layer 8, an i-In_(0.8)Ga_(0.2)As layer 9 (500Å thick), a C-doped p-GaAs layer 10 (carrier concentration: 4×10¹⁹ cm⁻³,thickness: 1,000 Å), an n-In_(0.5)Ga_(0.5)P layer 11 (carrierconcentration: 3×10¹⁷ cm⁻³, thickness: 1,000 Å), and an i-GaAs layer 12(200 Å thick), which are all formed on the GaAs substrate 7 in thatorder.

[0037] In FIGS. 2A and 2B, the p-GaAs layers 4 and 10 in which carbon isdoped as a p-type impurity correspond to the base layers of the HBTs. Inthe semiconductor crystal substrate shown in FIG. 2A (hereinafterreferred to as Sample I), the concentration of the hydrogen contained inthe p-GaAs layer 4 is 1×10¹⁹ cm⁻³. In the semiconductor crystalsubstrate shown in FIG. 2B (hereinafter referred to as Sample II), onthe other hand, the concentration of the hydrogen contained in thep-GaAs layer 10 is 4×10¹⁸ cm⁻³. The impurity concentrations of thep-GaAs layers 4 and 10 in Sample I and Sample II, respectively, are both4×10¹⁹ cm⁻³. Furthermore, the thicknesses of the p-GaAs layers 4 and 10are both approximately 1,000 Å, which is approximately equal to thethicknesses of the base layers of the HBTs.

[0038]FIG. 3A and FIG. 3B show PL intensities measured at roomtemperature using an Ar ion laser (light) having a wavelength of 488 nmas an excitation light source. FIG. 3A shows PL intensities of Sample Ishown in FIG. 2A, while FIG. 3B shows PL intensities of Sample II shownin FIG. 2B. In each figure, the horizontal axis indicates the excitationtime, and the vertical axis indicates the PL intensity. In FIG. 3A andFIG. 3B, the PL wavelength is 897 nm, which corresponds to the forbiddenenergy band gap of GaAs. The value of the PL intensity increases withtime and becomes constant from a certain time point, reaching thesaturation point. The change in the PL intensity of Sample I from thestart of the measurement to the saturation point is larger than that forSample II. That is, the higher the concentration of the hydrogencontained in the base layer, the longer the time required for the PLintensity to reach its saturation point. Furthermore, regardless of thehydrogen concentration, the PL intensity increases with increasingexcitation light intensity.

[0039] A description will be made below of the relationship between thebase current and the PL intensity. FIGS. 4A, 4B, and 4C each shows thetime dependences of the base current and the PL intensity of asemiconductor crystal substrate whose base layer has a differenthydrogen concentration. Specifically, FIG. 4A shows the time dependencesof the base current and the PL intensity with the hydrogen concentrationset to 1×10 ¹⁹ cm⁻³; FIG. 4B shows the time dependences with thehydrogen concentration set to 4×10¹⁸ cm⁻³; and FIG. 4C shows the timedependences with the hydrogen concentration set to 1×10¹⁸ cm⁻³.Furthermore, the base currents are measured with the collector-emittervoltage and the base-emitter voltage fixed to 2.5 V and 1.3 V,respectively. The PL intensities, on the other hand, are measured atroom temperature with the excitation light intensity set toapproximately 3.8 kW/cm² using an Ar ion laser (light) having awavelength of 488 nm. The changes in the intensity of the PL wavelength(λ=897) were plotted. As shown in the figures, as the concentration ofthe hydrogen contained in the base layer becomes higher, the changes inthe base current and in the PL intensity increase and the time requiredfor the base current and the PL intensity to saturate also increases.Therefore, the change in the PL intensity of a semiconductor crystalsubstrate over time can be measured to determine the change in the basecurrent over time, that is, the change in the current gain of the HBTdevice over time.

[0040] Incidentally, the hydrogen concentration of the base layer of anHBT is decided by the base layer growth conditions. Therefore, the PLintensity may be measured before actually manufacturing the device, andthe base layer growth conditions may be determined based on themeasurements to control the quality of the device. Conventionally, adevice is actually produced to measure its base current. The productionof the device takes at least approximately half a day. The qualitycontrol by use of the above PL intensity measurement, on the other hand,does not require the production of the device, and furthermore the PLintensity measurement itself takes only a few minutes, which leads to asignificant reduction in the entire working hours. Furthermore, thepresent invention inspects a semiconductor crystal substrate instead ofthe actual HBT device, making it possible to carry out nondestructiveinspection of the HBT device to measure its electrical properties.

[0041] For example, assume that the PL intensity of a semiconductorcrystal substrate is measured with an excitation light intensity of 3.8kW/cm² using an Ar ion laser (light) having an excitation wavelength of488 nm. Letting the PL saturation intensity value (the value of the PLintensity when it no longer changes with time in FIG. 3A˜3B, or FIGS.4A˜4C) be 1, if the value of the measured PL intensity reaches 0.95 ormore within 50 seconds from the start of the measurement, the initialchange in the current gain will be within 5%, which means that thesemiconductor crystal substrate is suitable for manufacture of a device.

[0042] It should be noted that the relationship between the current gainand the PL intensity of an HBT is described in Japanese PatentApplication Laid-open No. Hei 3-64943. The patent utilizes thecorrelations among the PL intensity, the carrier lifetime, and thecurrent gain, and measures the lifetime of the PL after the saturationof the PL intensity in order to measure the lifetime of the carriers inthe base layer. However, the lifetime of PL is generally on the order ofa few tens of picoseconds. This means that the above literature onlymeasures such a short time to obtain the lifetime of specific PL (andthe lifetime of the carriers in the base layer).

[0043] The present invention, on the other hand, is characterized inthat it utilizes the correlations among the current gain change, thebase current change, and the PL intensity change over time.Specifically, the present invention aims to measure how the PL intensity(which indicates the lifetime of the PL) changes in units of a few tensof seconds before its saturation, instead of measuring the lifetime ofthe PL itself after the saturation of the PL intensity. Therefore, thereis no need for measuring time-resolved PL on the order of picoseconds;it is only necessary to monitor the change in the PL intensity with timeon the order of seconds.

[0044] First Embodiment

[0045] The present embodiment characteristically uses the semiconductorcrystal substrate shown in FIG. 5 as a sample and measures its PLintensity. It should be noted that the term “sample” hereinafterindicates a semiconductor crystal substrate to be evaluated. Asemiconductor crystal substrate to be evaluated includes a crystal layercorresponding to a base layer used for manufacturing an actual HBT.

[0046] In an actual HBT, since the emitter layer, the contact layer,etc. are formed on the base layer, PL emitted from the base layer isabsorbed by these layers and as a result, PL of a low intensity can beonly observed. To solve this problem, the present invention uses asample made up of a GaAs substrate 13, an undoped GaAs layer 14, and ap-GaAs layer 15 doped with carbon as a p-type impurity. The undoped GaAslayer 14 and the p-GaAs layer 15 are formed on the GaAs substrate 13 inthat order. Alternatively, the p-GaAs layer 15 may be directly formed onthe GaAs substrate 13. In the present embodiment, the p-GaAs layer 15corresponds to the base layer of the HBT, and light emitted from thislayer is observed to measure the time dependence of the PL intensity.Since the present invention does not form any other layer on the layercorresponding to the base layer, it is possible to reduce the absorptionof PL by other layers, resulting in measurement with sufficientintensity. Furthermore, since the configuration of the sample is verysimple, it can be easily produced at low cost.

[0047] A sample of the present embodiment can be produced, for example,through epitaxial growth using the MOCVD method. The carrierconcentration of the p-GaAs layer 15 is preferably set to approximatelyfrom 1×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³. Its thickness is preferably set toapproximately from 500 Å to 10,000 Å. A thickness thinner than 500 Åresults in low PL intensity, while a thickness thicker than 10,000 Åleads to high cost. Both cases are undesirable.

[0048] The wavelength (λ) of excitation light used for the PLmeasurement is preferably set to from 300 nm to 550 nm. For example, anAr ion laser (light) having a wavelength of 488 nm may be used for themeasurement. On the other hand, since the main wavelength (λ) of PL atroom temperature (25° C.) is from 890 nm to 900 nm, it is desirable touse this wavelength to monitor the PL intensity.

[0049] Second Embodiment

[0050] The present embodiment characteristically uses the semiconductorcrystal substrate shown in FIG. 6 as a sample and measures its PLintensity. A p-GaAs layer 18 doped with carbon as a p-type impuritycorresponds to the base layer of the HBT. The present invention ischaracterized in that barrier layers 19 and 17 are formed over and underthe p-GaAs layer 18, respectively. As used herein, the term “barrierlayer” is a layer which functions to confine excited minority carriersand thereby increase the PL intensity. A material to be used for abarrier layer must have a forbidden energy band gap larger than that ofthe p-GaAs layer. That is, another semiconductor layer having aforbidden energy band gap larger than that of the p-GaAs layer is bondedto each of the top and the bottom surfaces of the p-GaAs layer so thatan energy barrier can be formed due to the difference between theseforbidden energy band gaps. The formation of this energy barrier makesit difficult for the carriers within the p-GaAs layer (that is, theelectrons and holes) to leave the layer, confining them therein. Withthis arrangement, the electrons and the holes in the base later can beefficiently recombined together, making it possible to increase the PLintensity.

[0051] Materials such as In_(0.5)Ga_(0.5)P and Al_(0.3)Ga_(0.7)As can beused for the barrier layers for the present embodiment. These materialsmay be doped or undoped. Further, the barrier layers formed over andunder the p-GaAs layer may be made of the same material or differentmaterials. Still further, barrier layers need not be formed both overand under the p-GaAS layer. A barrier layer may be formed only eitherover or under the p-GaAs layer.

[0052] A sample of the present embodiment can be produced, for example,through epitaxial growth using the MOCVD method. The carrierconcentration of the p-GaAs layer 18 is preferably set to approximatelyfrom 1×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³. Its thickness is preferably set toapproximately from 500 Å to 10,000 Å. A thickness thinner than 500 Åresults in low PL intensity, while a thickness thicker than 10,000 Åleads to high cost. Both cases are undesirable.

[0053] Unlike the first embodiment, the present embodiment ischaracterized in that the barrier layers 19 and 17 are formed over andunder the p-GaAs layer 18, respectively. Therefore, the thicknesses ofthe barrier layers 17 and 19 are preferably each set to approximatelyfrom 100 Å to 1,000 Å to reduce the absorption by the barrier layers 17and 19 of PL emitted from the p-GaAs layer 18.

[0054] The wavelength (λ) of excitation light used for the PLmeasurement is preferably set to from 300 nm to 550 nm. For example, anAr ion laser (light) having a wavelength of 488 nm may be used for themeasurement. On the other hand, since the main wavelength (λ) of PL atroom temperature (25° C.) is from 890 nm to 900 nm, it is desirable touse this wavelength to monitor the PL intensity.

[0055] Third Embodiment

[0056] The present embodiment characteristically uses the semiconductorcrystal substrate shown in FIG. 7 as a sample and measures its PLintensity. That is, the present embodiment is characterized in that itmeasures a sample having the same configuration as that of thesemiconductor crystal substrate constituting an actual HBT. Therefore,according to the present embodiment, a sample HBT device can be actuallyproduced from a measured sample (semiconductor crystal substrate),making it possible to obtain accurate information on the electricalcharacteristics of an HBT device to be produced by using thesemiconductor crystal substrate beforehand. Furthermore, since theformation of the base layer of an actual HBT is affected by latticedefects in the crystal layers formed under the base layer, the use of asample according to the present embodiment having the same configurationas that of an actual semiconductor crystal substrate makes it possibleto carry out more accurate evaluation.

[0057] As shown in FIG. 7, a semiconductor crystal substrate accordingto the present embodiment comprises a GaAs substrate 20, and an n⁺-GaAslayer 21, an n-GaAs layer 22, a p-GaAs layer 23, an n-barrier layer 24,and an n-GaAs layer 25, which are all formed on the GaAs substrate 20 inthat order. The p-GaAs layer 23 is doped with carbon as a p-typeimpurity. These layers can be formed through epitaxial growth using theMOCVD method. The carrier concentration of the p-GaAs layer 23 ispreferably set to from 1×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³, and its thickness ispreferably set to approximately from 500 Å to 10,000 Å. A thicknessthinner than 500 Å results in low PL intensity, while a thicknessthicker than 10,000 Å leads to high cost. Both the cases areundesirable. On the other hand, the carrier concentration of the n⁺-GaAslayer 21 is preferably set to 1×10¹⁸ cm⁻³ or more, and its thickness ispreferably set to 500 Å or less. Materials such as In_(0.5)Ga_(0.5)P andAl_(0.3)Ga_(0.7)As can be used for the n-barrier layer 24. The carrierconcentration of the n-barrier layer 24 is preferably set to from 1×10¹⁷cm⁻³ to 5×10¹⁷ cm⁻³, and its thickness is preferably set toapproximately from 100 Å to 500 Å. Furthermore, the carrierconcentrations of the n-GaAs layers 22 and 25 are preferably set to1×10¹⁷ cm⁻³ or less, and their thicknesses are preferably set to 2,000 Åor more.

[0058] The wavelength (λ) of excitation light used for the PLmeasurement is preferably set to from 300 nm to 550 nm. For example, anAr ion laser (light) having a wavelength of 488 nm may be used for themeasurement. On the other hand, since the main wavelength (λ) of PL atroom temperature (25° C.) is from 890 nm to 900 nm, it is desirable touse this wavelength to monitor the PL intensity.

[0059] Fourth Embodiment

[0060] The present embodiment characteristically uses the semiconductorcrystal substrate shown in FIG. 8 as a sample and measures its PLintensity. A p-GaAs layer 29 doped with carbon as a p-type impuritycorresponds to the base layer of the HBT. The present invention ischaracterized in that a barrier layer 28 is formed under the p-GaAslayer 29. A material to be used for a barrier layer must have aforbidden energy band gap larger than that of the p-GaAs layer. Withthis arrangement, an energy barrier is produced due to the differencebetween these forbidden energy band gaps, making it possible to confinethe carriers within the base layer so that the electrons and the holescan be efficiently recombined together, increasing the intensity of PLfrom the base layer.

[0061] Further, since the configuration of the sample (semiconductorcrystal substrate) of the present embodiment is similar to that of thesemiconductor crystal substrate of an actual HBT device, it is possibleto obtain accurate information on the electrical characteristics of theactual HBT device to be produced by using the sample semiconductorcrystal substrate beforehand. Furthermore, since the formation of thebase layer of the actual HBT is affected by lattice defects in thecrystal layers formed under the base layer, the present embodiment makesit possible to carry out more accurate evaluation also in this respect.

[0062] As shown in FIG. 8, a semiconductor crystal substrate accordingto the present embodiment comprises a GaAs substrate 26, and an n⁺-GaAslayer 27, an n-barrier layer 28, a p-GaAs layer 29, an n-barrier layer30, and an n-GaAs layer 31, which are all formed on the GaAs substrate26 in that order. The p-GaAs layer 29 is doped with carbon as a p-typeimpurity. These layers can be formed through epitaxial growth using theMOCVD method. The carrier concentration of the p-GaAs layer 29 ispreferably set to approximately from 1×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³, and itsthickness is preferably set to approximately from 500 Å to 10,000 Å. Athickness thinner than 500 Å results in low PL intensity, while athickness thicker than 10,000 Å leads to high cost. Both the cases areundesirable. On the other hand, the carrier concentration of the n⁺-GaAslayer 27 is preferably set to 1×10¹⁸ cm⁻³ or more, and its thickness ispreferably set to 500 Å or less. Furthermore, the carrier concentrationof the n-GaAs layers 28 and 31 are preferably set to 1×10¹⁷ cm⁻³ orless, and their thicknesses are preferably set to 2,000 Å or more.

[0063] Materials such as In_(0.5)Ga_(0.5)P and Al_(0.3)Ga_(0.7)As can beused for the barrier layers for the present embodiment. The barrierlayers formed over and under the C-doped GaAs layer may be made of thesame material or different materials. Further, the carrierconcentrations of the barrier layers are preferably set to from 1×10¹⁷cm⁻³ to 5×10¹⁷ cm⁻³, and their thicknesses are preferably set toapproximately from 100 Å to 500 Å.

[0064] The wavelength (λ) of excitation light used for the PLmeasurement is preferably set to from 300 nm to 550 nm. For example, anAr ion laser (light) having a wavelength of 488 nm may be used for themeasurement. On the other hand, since the main wavelength (λ) of PL atroom temperature (25° C.) is from 890 nm to 900 nm, it is desirable touse this wavelength to monitor the PL intensity.

[0065] The features and advantages of the present invention may besummarized as follows.

[0066] According to one aspect, it is possible to determine the changein the base current of an HBT with time, that is, the change in thecurrent gain of the HBT with time, by measuring the change in the PLintensity of a semiconductor crystal substrate with time.

[0067] According to another aspect, since no other layer is formed on alayer corresponding to a base layer, it is possible to reduce theabsorption of PL by other layers, resulting in measurement withsufficient intensity.

[0068] According to another aspect, it is possible to efficientlyrecombine electrons and holes together within a base layer, resulting inincreased PL intensity.

[0069] According to another aspect, since a sample HBT device can beactually produced from a measured sample (semiconductor crystalsubstrate) it is possible to obtain accurate electrical information onan HBT device to be produced by using the semiconductor crystalsubstrate beforehand.

[0070] According to another aspect, it is possible to efficientlyrecombine electrons and holes together within a base layer, resulting inincreased PL intensity. Furthermore, since a sample HBT device can beactually produced from a measured sample (semiconductor crystalsubstrate), it is possible to obtain accurate electrical information onan HBT device to be produced by using the semiconductor crystalsubstrate beforehand.

[0071] According to another aspect, it is possible to efficientlyrecombine electrons and holes together within a base layer, resulting inincreased PL intensity.

[0072] According to other aspect, it is possible to determine the changein the base current of an HBT device with time, that is, the change inthe current gain of the HBT device with time, by measuring the change inthe intensity of PL from a p-type GaAs crystal layer doped with carbonwith time.

[0073] Obviously many modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that within the scope of the appended claimsthe invention may by practiced otherwise than as specifically described.

[0074] The entire disclosure of a Japanese Patent Application No.2002-228514, filed on Aug. 6, 2002 including specification, claims,drawings and summary, on which the Convention priority of the presentapplication is based, and incorporated herein by reference in itsentirety.

What is claimed is:
 1. A method for evaluating a semiconductor crystalsubstrate which includes a collector layer, a base layer, and an emitterlayer and is used for a heterojunction bipolar transistor, said methodcomprising the steps of: producing a semiconductor crystal substrate tobe evaluated which includes a crystal layer whose composition is thesame as that of said base layer; irradiating excitation light to saidsemiconductor crystal substrate to be evaluated and measuring a changewith time in an intensity of photoluminescence from said crystal layerbefore said intensity becomes saturated; and estimating a change withtime in a current gain of said heterojunction bipolar transistorproduced using said semiconductor crystal substrate based on said changewith time in said intensity.
 2. The method for evaluating asemiconductor crystal substrate according to claim 1, wherein said baselayer is a p-type GaAs crystal layer containing carbon, saidsemiconductor crystal substrate to be evaluated includes a GaAssubstrate and a p-type GaAs crystal layer containing carbon, and awavelength of said excitation light is 300 nm to 550 nm.
 3. The methodfor evaluating a semiconductor crystal substrate according to claim 2,wherein said semiconductor crystal substrate to be evaluated includes anundoped GaAs crystal layer formed between said GaAs substrate and saidp-type GaAs crystal layer containing carbon.
 4. The method forevaluating a semiconductor crystal substrate according to claim 2,wherein said semiconductor crystal substrate to be evaluated includes abarrier layer formed either over or under said GaAs crystal layercontaining carbon, or both.
 5. The method for evaluating a semiconductorcrystal substrate according to claim 4, wherein a thickness of saidbarrier layer is 100 Å to 1,000 Å.
 6. The method for evaluating asemiconductor crystal substrate according to claim 4, wherein said eachbarrier layer is an InGaP crystal layer or an AlGaAs crystal layer. 7.The method for evaluating a semiconductor crystal substrate according toclaim 2, wherein a carrier concentration of said p-type GaAs crystallayer containing carbon is 1×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³, and a thicknessof said p-type GaAs crystal layer containing carbon is 500 Å to 10,000Å.
 8. The method for evaluating a semiconductor crystal substrateaccording to claim 1, wherein said base layer is a p-type GaAs crystallayer containing carbon; said semiconductor crystal substrate to beevaluated includes a GaAs substrate and an n⁺-type GaAs crystal layer,an n-type GaAs crystal layer, a p-type GaAs crystal layer containingcarbon, an n-type barrier layer, and an n-type GaAs layer, these layersbeing formed on said GaAs substrate in that order; and a wavelength ofsaid excitation light is 300 nm to 550 nm.
 9. The method for evaluatinga semiconductor crystal substrate according to claim 8, wherein acarrier concentration of said n-type barrier layer is 1×10¹⁷ cm⁻³ to5×10¹⁷ cm⁻³, and a thickness of said n-type barrier layer is 100 Å to500 Å.
 10. The method for evaluating a semiconductor crystal substrateaccording to claim 8, wherein said barrier layer is an InGaP crystallayer or an AlGaAs crystal layer.
 11. The method for evaluating asemiconductor crystal substrate according to claim 8, wherein a carrierconcentration of said p-type GaAs crystal layer containing carbon is1×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³, and a thickness of said p-type GaAs crystallayer containing carbon is 500 Å to 10,000 Å.
 12. The method forevaluating a semiconductor crystal substrate according to claim 1,wherein said base layer is a p-type GaAs crystal layer containingcarbon; said semiconductor crystal substrate to be evaluated includes aGaAs substrate and an n⁺-type GaAs crystal layer, an n-type barrierlayer, a p-type GaAs crystal layer containing carbon, an n-type barrierlayer, and an n-type GaAs layer, these layers being formed on said GaAssubstrate in that order; and a wavelength of said excitation light is300 nm to 550 nm.
 13. The method for evaluating a semiconductor crystalsubstrate according to claim 12, wherein a carrier concentration of saidn-type barrier layer is 1×10¹⁷ cm⁻³ to 5×10¹⁷ cm⁻³, and a thickness ofsaid n-type barrier layer is 100 Å to 500 Å.
 14. The method forevaluating a semiconductor crystal substrate according to claim 12,wherein said barrier layer is an InGaP crystal layer or an AlGaAscrystal layer.
 15. The method for evaluating a semiconductor crystalsubstrate according to claim 12, wherein a carrier concentration of saidp-type GaAs crystal layer containing carbon is 1×10¹⁸ cm⁻³ to 1×10²⁰cm⁻³, and a thickness of said p-type GaAs crystal layer containingcarbon is 500 Å to 1,000 Å.